The present invention relates to power control circuits, and more particularly relates to circuit breakers providing overload and short circuit protection.
Presently available circuit breakers of the electromagnetic or thermal type are known to have turn off times in the range of 10 milliseconds or more. This 10 millisecond turn off time is dangerously too long in many critical circuit breaker applications, such as military, flammable or explosive environments. Furthermore, in large electrical systems, the tripped circuit breaker must be located and subsequently re-set manually in order to restore the circuit after the fault has been cleared. These problems of long turn off times and manual resetting associated with mechanical circuit breakers may be disadvantageous and very serious in applications where time is a critical factor.
The Triac is a conventional electronic power device made of silicon crystal comprising a dual sense silicon controlled rectifier (SCR) for receiving a gate signal for controlling power through a load, such as in large wattage dimmer devices. One problem associated with SCR or Triac devices in power control applications is that once the Triac is turned on, it will not turn off unless the supply voltage or current is cut off, or else turn off will not occur until the next zero crossing of the A.C. power supply. This time duration required to turn off the Triac may be as much as one half of the A.C. cycle, resulting in damaging the Triac under overload and short circuit conditions.
In the U.S. Pat. No. 4,633,161 issued to Callahan et al on Dec. 30, 1986, there is described an inductorless phase control dimmer power controller for coupling a lamp to an alternating current source. This patent is directed to the elimination of the filter inductor from the power stage of the conventional solid state electronic dimmer. Here, in this dimmer controller, the main power is dissipated in the diodes of a pair of power MOSFET due to the forward voltage drop across the FET diodes which are used as the current conducting path for the dimmer load. This results in excessively high power dissipation in the FET devices due to high voltage/high current operation in the linear mode. Thus, such inductorless dimmer power controller may be used in lower power dimmer applications, but would dissipate too much heat and burn out if employed in the higher power control circuits. This burn out occurs since the internal clamp diode, being located in the same case as the MOSFET, increases in temperature, and consequently increases the MOSFET's RDS/on, thereby increasing the dissipation on the MOSFET itself. Also, the power dissipation in the patented power control device is increased across the MOSFET since the load current rise time is increased for the purpose of eliminating the EMI filter inductor by forcing it to operate in the linear mode. Thus, by turning the FETS on slowly, as occurs when turning on the MOSFET as half cycle in the linear mode, large heat dissipation in the MOSFET will result. Furthermore, the absence of the inductor will allow fast current rises in the case of a short circuit or overload condition, faster than the response of the current limit circuit. The only element that controls the current slew rate is the line inductance. If the fault occurs at 1/4 cycle, where the line voltage equals the plus or minus peak value, and the electric line to the load is short, the efficiency of the current limit circuit is questionable in view of the maximum allowed load current in the linear mode. This also contributes to the high heat dissipation in the MOSFET power control device.
In view of the above, it is an object of the present invention to provide an A.C. solid state circuit breaker with overload and short circuit protection. It is another object to provide an A.C. solid state circuit breaker with a very fast turn off time, in the range of microseconds. It is another object to provide an A.C. solid state circuit breaker which is resistant to short circuit destruction which destroys conventional solid state circuit breakers and power controllers, and which can operate in very high power ranges, such as with currents in excess of 100 amperes. It is another object to provide an A.C. solid state circuit breaker having very low power dissipation in the power control device and which operates at fast speeds.